1. Technical Field
This invention relates generally to the field of gas dryers. In particular, this invention relates to a desiccant gas dryer of the kind having a pair of sorbent or desiccant beds, wherein one bed is regenerated as the other bed adsorbs vapor.
2. Description of the Related Art
The desirability of drying air after compression is well known. A variety of systems have been developed over the years for this purpose, many of which employ two beds containing an adsorbent or desiccant material such as activated alumina. In such systems the beds alternately dry the process stream and then are regenerated either using an auxiliary source of heat in so-called heat-reactivated systems, or through conservation and use of the heat of adsorption for effecting regeneration, in so-called heaterless pressure-swing systems. In heat-reactivated systems, it has been common to use fixed cycles several hours in length, such that a bed is on adsorption for a period of time, perhaps eight hours, after which it is regenerated during the eight hour adsorption cycle for the other bed. In the heaterless systems relatively short, fixed cycle times are used, typically about four minutes for drying with four minutes allowed for regeneration while the other bed is drying the process stream. Mechanical cam timers operating solenoids which in turn operate the flow control valves have been employed for the control of such fixed cycle systems.
Typically both heat-reactivated and heaterless systems are sized for maximum contemplated loading conditions, conditions which may seldom or never be encountered during actual operation. As a result, at the end of a fixed drying cycle a bed may have adsorbed only a fraction of its moisture capacity. In the case of heat-reactivated dryers, such underutilization of the bed while on adsorption coupled with a full energy regeneration can seriously affect overall operating efficiency.
Over the years "demand" rather than fixed cycling has been employed in both heat-reactivated and heaterless dryers. Seibert, et al. U.S. Pat. No. 3,448,561 discloses a system for sensing the actual moisture content of the on-stream bed and terminating the adsorption cycle when regeneration is complete. Alternatively, the patent suggests that the moisture content of the regenerated bed can be monitored, and the purge flow can be terminated when the moisture content of the bed has dropped below a predetermined minimum. That patent discloses the use of a lithium chloride humidity sensor external to the desiccant beds with gas sampling conduit leading from the beds to the cell.
A fail-safe mechanism for the system of Seibert et al. is described by McKey et al. U.S. Pat. No. 4,127,395. To ensure continued operation of the dryer at an acceptable time cycle despite contamination of the humidity sensing element, failure of the sensing element is detected and in response thereto the dryer is automatically placed in a fixed cycle mode of operation. A failed sensor element is detected by introducing to the sensor from time to time, at an interval short enough to ensure that the desiccant bed or stream has not been exhausted, wet air from the dryer inlet.
A brute-force technique for demand control of heaterless dryers is disclosed by White, Jr. et al. U.S. Pat. No. 4,197,095. White, Jr. et al. teach that the flow rate, inlet and outlet temperatures, inlet and outlet pressures, and regenerating pressures could be sensed, and all the sensed information should be fed to a microprocessor programmed to calculate the quantity of purge and the purge flow rate, and based on these calculations, control the regeneration and cycling time. Such a control method, however, has had limited commercial application due to the relatively high cost of reliable sensors and their associated electronics.
To date, considerable success has been achieved in sensing the moisture loading on a desiccant bed by placing a capacitor directly into the bed with the sorbent or desiccant between the capacitor plates. As the moisture loading varies, the dielectric constant of the sorbent changes, such that the capacitance provides an electrical indication of the moisture loading. The use of such a capacitor probe for the control of heat-reactivated and heaterless dryers is disclosed in Gravatt [U.S. patent application Ser. No. 577,728, now U.S. Pat. No. 4,552,570].
A microcomputer-based control system for a desiccant dryer employing the capacitance probe of Gravatt is described in Tinker [U.S. patent application Ser. No. 444,502, filed Nov. 24, 1982 and now U.S. Pat. No. 4,546,442] which is herein incorporated by reference. The specific example described by Tinker is a control for a heaterless dryer. The control has an alarm relay and indicators for signalling inlet and exhaust valve malfunctions and probe failures.
Heat-reactivated adsorbers have been used for purifying as well as drying gasses. McGee, U.S. Pat. No. 4,422,859 discloses a heat-reactivated adsorber for purifying hydrogen gas. To prevent explosions, the flow of hydrogen gas and the heater are shut off in response to sensing the hydrogen gas concentration and temperature just outside of the adsorber.
Microcomputer-based systems are increasingly being used in lieu of mechanical cam or sequence drum controllers. In order to emulate such mechanical controls, the microcomputer must recover from a power failure so as to resume the control sequence at the point where power was lost. A microcomputer-based system having this feature is disclosed by Morley et al., U.S. Pat. No. 4,213,174. Power-down, power-up circuitry is said to insure an ordered and complete shutdown of the controller if any number of conditions exist, including utility AC failure and impending failure of several of the power supply voltages. Handshaking circuitry between the power supply and the remainder of the controller is said to insure that the controller maintains memory validity for all types of shutdown situations, including momentary loss of any supply voltage. The handshaking circuitry includes a watchdog circuit in FIG. 12B.